Plasma spraying process

ABSTRACT

The application relates to a process for producing a coating by thermal spraying, in particular by plasma spraying, in which a component, in particular a cylinder liner, is internally coated with an alloy. It is proposed that nitrogen is fed as transporting gas, a spraying additive being a solid alloy wire which is guided into a plasma stream, and coating being performed without additional powder.

The present invention relates to a process for producing a coating bythermal spraying, in particular by plasma spraying, in which acomponent, in particular a cylinder liner of an internal combustionengine, which is produced for example from aluminum, is coated with analloy, preferably with an iron alloy.

It is known from EP 1 967 601 A2 to coat, for example, an aluminumengine block, in particular the cylinder bearing surface thereof, withan iron alloy by carrying out arc wire spraying. In this respect, EP 1967 601 A2 proposes the use of an iron alloy which contains, inter alia,5 to 25% by weight chromium. It is essential in the case of EP 1 967 601A2 that an additional powder, to be precise boron carbide, isadditionally fed to the iron melt. The arc wire spraying process of EP 1967 601 A2 involves what is known as the TWAS process, in which twowires are fed to a spray head in such a manner that the power istransmitted across the wires. If the two wires make contact, an arcwhich melts the wires is formed by a permanent short circuit. A nozzlefrom which compressed air or an inert gas such as nitrogen is dischargedis located downstream of the nozzle. This gas stream atomizes the molteniron alloy and feeds it together with the molten boron carbide powder tothe surface to be coated.

DE 44 11 296 A1 and DE 44 47 514 A1 are concerned with coatings providedby means of plasma spraying, in which however a metal powder or a fillerwire are melted and in which nitrogen is fed to the material mixture bymeans of metallic nitrogen compounds in order to harden the coating.

Present-day internal combustion engines and the engine blocks thereofcan be cast from a metal or aluminum, aluminum blocks in particularhaving an iron or metal layer on the cylinder bores thereof. The metallayer can be sprayed on by thermal processes. The processes mentionedabove are known as thermal spraying processes.

What is termed the PTWA (Plasma Transfer Wire Arc) internal coatingprocess is also known. In this process, bores (cylinder bores) can becoated from the inside with a wire-like spraying additive. Here,therefore, only a single wire-like spraying additive is supplied, itbeing possible to use a filler wire or else to supply spraying powder.The plasma impinges on the preheated, wire-like spraying additive. Theplasma gas is usually an argon-hydrogen mixture. In the PTWA process,air or compressed air is used as the transporting gas or atomizer gas.The layers which are produced by this process are distinguished by a lowporosity. The PTWA internal coating process has proven suitable to datefor the internal coating of cylinder bores.

However, it has been found that the metal or iron coatings produced bythe coating processes possible to date for the cylinder bores do notwithstand the particular corrosion conditions of ethanol-containingfuels or ethanol fuels. This is observed particularly when the motorvehicle or the internal combustion engine is not used for a relativelylong time, which may be the case for example when it is switched offduring a vacation. Even an alloy containing 17% by weight chromium hadtraces of corrosive attack on the protective coating.

Proceeding from the identified problem of corrosion caused byethanol-containing fuels on metal coatings of cylinder bores, theinvention is based on the object of specifying a process of the typementioned in the introduction which makes it possible to produce acoating which is improved in this respect.

According to the invention, the object is achieved by a process havingthe features of claim 1. Further particularly advantageous refinementsof the invention are disclosed in the subclaims.

It is pointed out that the features specified individually in thefollowing description may be combined with one another in any desiredtechnically meaningful way and disclose further refinements of theinvention.

The invention proposes a process for producing a coating by thermalspraying, in particular by plasma spraying, in which a component, inparticular a cylinder liner of an internal combustion engine, which isproduced or cast from aluminum, is coated with an alloy, in whichprocess nitrogen is fed at least as transporting gas, a sprayingadditive being a solid alloy wire which is guided into a plasma stream,and coating being performed without additional powder or without powder.It is advantageous if the plasma spraying is PTWA (Plasma Transfer WireArc) internal coating.

Within the context of the invention, the term “without additionalpowder” or “without powder” means that neither a filler wire filled with(metal) powder nor a separately fed (metal) powder is used.Specifically, in the invention, use is advantageously made merely of asolid, i.e. homogeneous or an unfilled additional spraying wire. Asuitable alloy for coating comprises chromium as alloying element andmainly iron. A preferred metal or iron alloy is disclosed further below.

On account of the advantageous use of nitrogen gas as transporting gasinstead of air or instead of compressed air, as is used for example inthe known PTWA process, the transformation or the degradation of thechromium caused by the oxygen in the air used to date is precluded, andtherefore the entire chromium proportion of the alloy can be used toform a stable protective layer. On account of the difference in the freeenthalpy (or the Gibbs free energy), predominantly aluminum nitridesrather than chromium nitrides are formed. These aluminum nitridesreplace the existing wear-resistant metal oxides which form during thePTWA spraying process with compressed air. As a result, the inventionthus gives rise not only to a wear-resistant coating, but also to aprotective (anti-corrosion) layer which withstands the corrosive attacksin particular by ethanol-containing fuels. Within the context of theinvention, ethanol-containing fuels for internal combustion engines cancontain conventional fossil fuel (e.g. E5, E10 or E85) as an admixtureof ethanol, or can be used in pure form (E100).

In the PTWA process known to date, it was assumed that an alloycomprising 17% by weight Cr is particularly suitable for meeting therequirements. By contrast, it is expediently provided in the inventionthat the additional spraying wire comprises an iron alloy with achromium proportion of 12 to 35% by weight. Further alloyingconstituents can be aluminum (2-10% by weight), silicon (0-1% byweight), manganese (0-1% by weight), carbon (0-1% by weight) and furtherconstituents such as, for example, phosphorus (0-1% by weight), sulfur(0-0.09% by weight), molybdenum (0-5% by weight), nickel (0-1% byweight), copper (0-0.5% by weight), nitrogen (0-0.5% by weight), theremainder being iron.

In a particularly preferred embodiment, the additional spraying wirecomprises, according to the invention, an iron alloy comprising 23% byweight Cr, 5% by weight Al, less than 0.5% by weight Si, less than 0.2%by weight Mn, less than 0.05% by weight C, the remaining constituentshaving a proportion of less than 2% by weight, and the remainder beingiron.

It goes without saying that the process according to the invention canalso be used for coating other components.

FIG. 1 shows a nozzle unit 1 of a PTWA internal coating apparatus. ThePTWA (Plasma Transferred Wire Arc) coating system is a system forcoating bores, in particular cylinders in engine blocks of internalcombustion engines. The nozzle unit 1 consists of a cathode 2, a plasmanozzle 3 and the electrically conductive alloy wire 4 as anode, which isfed perpendicularly to the plasma nozzle 3. The material used for thecathode 2 is preferably tungsten, which may also be doped with thorium,for example. The plasma gas 5, for example a mixture of argon andhydrogen, is fed through bores made in the nozzle body 6 and lyingtangentially to the circumference. The cathode holder 7 isolates thecathode 2 from the nozzle body 6. The alloy wire 4 is guided in the wirefeed 15 such that it can move in rotation and be displacedlongitudinally.

The process is started by a high-voltage discharge, which ionizes anddissociates the plasma gas 5 between alloy wire 4, nozzle body 6 andcathode 2. The thus produced plasma flows through the plasma nozzle 3 athigh speed. In the process, the plasma gas 5 is transported toward thealloy wire 4 fed continuously perpendicularly to the nozzle 3, as aresult of which the electric circuit is completed.

In addition, a transporting gas 9 or an atomizer gas 9 is fed via feedducts 10 and auxiliary nozzles 11 to the plasma jet 8 emerging from thepilot nozzle 3.

The melting and the atomization of the alloy wire 4 are influenced inthis case by two phenomena. The wire 4 is firstly resistance heated bylarge current intensities, which are typically 65-90 amperes. The impactof the plasma jet 8 on the preheated wire 4 ensures that the lattermelts at the wire end 12. In other words, a plasma is generated insidethe plasma nozzle 3 by means of high-voltage discharge. A targetednitrogen gas flow, i.e. the transporting gas 9, along the discharge pathtransports the plasma and the molten spraying material 13 onto thesurface 14 of the workpiece to be coated.

1. A process for producing a coating by thermal spraying including byplasma spraying, comprising: coating with an alloy a component, whereinnitrogen gas is fed at least as transporting gas, a spraying additivebeing a solid alloy wire which is guided into a plasma stream, andcoating being performed without additional powder.
 2. The process asclaimed in claim 1, wherein the plasma spraying is a PTWA coatingprocess, and wherein the component is a cylinder bearing surface.
 3. Theprocess as claimed in claim 1, wherein the additional spraying wire isan iron alloy comprising 12 to 35% by weight Cr, 2-10% by weight Al,0-1% by weight Si, 0-1% by weight Mn, 0-1% by weight C.
 4. The processas claimed in claim 3, wherein the iron alloy further comprisesconstituents including 0-1% by weight P, 0-0.09% by weight S, 0-5% byweight Mo, 0-1% by weight Ni, 0-0.5% by weight Cu, 0-0.5% by weight N,and a remainder Fe.
 5. The process as claimed in claim 1, wherein theadditional spraying wire is an iron alloy comprising 23% by weight Cr,5% by weight Al, less than 0.5% by weight Si, less than 0.2% by weightMn, less than 0.05% by weight C, and remaining constituents having aproportion of less than 2% by weight, a remainder being iron.
 6. Theprocess as claimed in claim 1, wherein the component is an internalcombustion engine component.
 7. The process as claimed in claim 6,wherein the component is cast aluminum.
 8. The process as claimed inclaim 6, wherein the transporting gas is air.
 9. The process as claimedin claim 6, wherein the solid alloy wire is homogeneous.
 10. The processas claimed in claim 6, further comprising operating the engine withethanol containing fuel.
 11. A process, comprising: producing a coatingby thermal spraying, including by plasma spraying, including coating anengine cylinder bearing surface with an alloy, where nitrogen gas is fedat least as transporting gas, a spraying additive being a solid alloywire which is guided into a plasma stream, and coating being performedwithout additional powder, wherein the plasma spraying is a PTWA coatingprocess.
 12. The process as claimed in claim 11, wherein the cylinderbearing surface is cast aluminum, the engine being configured to combustethanol-containing fuel.
 13. The process as claimed in claim 12, whereinthe transporting gas is air.
 14. The process as claimed in claim 13,wherein the solid alloy wire is homogeneous.
 15. The process as claimedin claim 14, further comprising operating the engine withethanol-containing fuel.